If user doesn't specify driver, don't validate against existing cluster

[priyawadhwa]

$ minikube start --driver docker
😄  minikube v1.9.0-beta.1 on Darwin 10.14.6
✨  Using the docker driver based on user configuration
🔥  Creating Kubernetes in docker container with (CPUs=2) (8 available), Memory=1988MB (1988MB available) ...
🐳  Preparing Kubernetes v1.18.0-rc.1 on Docker 19.03.2 ...
    ▪ kubeadm.pod-network-cidr=10.244.0.0/16
    > kubectl.sha256: 65 B / 65 B [--------------------------] 100.00% ? p/s 0s
    > kubeadm.sha256: 65 B / 65 B [--------------------------] 100.00% ? p/s 0s
    > kubelet.sha256: 65 B / 65 B [--------------------------] 100.00% ? p/s 0s
    > kubectl: 41.99 MiB / 41.99 MiB [---------------] 100.00% 1.72 MiB p/s 25s
    > kubeadm: 37.97 MiB / 37.97 MiB [---------------] 100.00% 1.50 MiB p/s 26s
    > kubelet: 108.02 MiB / 108.02 MiB [-------------] 100.00% 1.99 MiB p/s 55s
🚀  Launching Kubernetes ... 
🌟  Enabling addons: default-storageclass, storage-provisioner
⌛  Waiting for cluster to come online ...
🏄  Done! kubectl is now configured to use "minikube"

❗  /usr/local/bin/kubectl is v1.16.1, which may be incompatible with Kubernetes v1.18.0-rc.1.
💡  You can also use 'minikube kubectl -- get pods' to invoke a matching version

$ minikube start 
😄  minikube v1.9.0-beta.1 on Darwin 10.14.6
✨  Using the docker driver based on existing profile
⌛  Reconfiguring existing host ...
🏃  Using the running docker "minikube" container ...
🐳  Preparing Kubernetes v1.18.0-rc.1 on Docker 19.03.2 ...
    ▪ kubeadm.pod-network-cidr=10.244.0.0/16
🚀  Launching Kubernetes ... 
🌟  Enabling addons: default-storageclass, storage-provisioner
🏄  Done! kubectl is now configured to use "minikube"

❗  /usr/local/bin/kubectl is v1.16.1, which may be incompatible with Kubernetes v1.18.0-rc.1.
💡  You can also use 'minikube kubectl -- get pods' to invoke a matching version

Should fix #7095

[k8s-ci-robot]

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[codecov-io]

Codecov Report

Merging #7096 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7096      +/-   ##
==========================================
- Coverage   37.20%   37.19%   -0.01%     
==========================================
  Files         146      146              
  Lines        9104     9106       +2     
==========================================
  Hits         3387     3387              
- Misses       5301     5303       +2     
  Partials      416      416              

Impacted Files	Coverage Δ
cmd/minikube/cmd/start.go	37.69% <0.00%> (-0.13%)	⬇️